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kbuild: introduce ccflags-remove-y and asflags-remove-y

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CFLAGS_REMOVE_<file>.o filters out flags when compiling a particular
object, but there is no convenient way to do that for every object in
a directory.

Add ccflags-remove-y and asflags-remove-y to make it easily.

Use ccflags-remove-y to clean up some Makefiles.

The add/remove order works as follows:

 [1] KBUILD_CFLAGS specifies compiler flags used globally

 [2] ccflags-y adds compiler flags for all objects in the
     current Makefile

 [3] ccflags-remove-y removes compiler flags for all objects in the
     current Makefile (New feature)

 [4] CFLAGS_<file> adds compiler flags per file.

 [5] CFLAGS_REMOVE_<file> removes compiler flags per file.

Having [3] before [4] allows us to remove flags from most (but not all)
objects in the current Makefile.

For example, kernel/trace/Makefile removes $(CC_FLAGS_FTRACE)
from all objects in the directory, then adds it back to
trace_selftest_dynamic.o and CFLAGS_trace_kprobe_selftest.o

The same applies to lib/livepatch/Makefile.

Please note ccflags-remove-y has no effect to the sub-directories.
In contrast, the previous notation got rid of compiler flags also from
all the sub-directories.

The following are not affected because they have no sub-directories:

  arch/arm/boot/compressed/
  arch/powerpc/xmon/
  arch/sh/
  kernel/trace/

However, lib/ has several sub-directories.

To keep the behavior, I added ccflags-remove-y to all Makefiles
in subdirectories of lib/, except the following:

  lib/vdso/Makefile        - Kbuild does not descend into this Makefile
  lib/raid/test/Makefile   - This is not used for the kernel build

I think commit 2464a609de ("ftrace: do not trace library functions")
excluded too much. In the next commit, I will remove ccflags-remove-y
from the sub-directories of lib/.

Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Brendan Higgins <brendanhiggins@google.com> (KUnit)
Tested-by: Anders Roxell <anders.roxell@linaro.org>
This commit is contained in:
Masahiro Yamada
2020-07-07 18:21:16 +09:00
parent 3ec8a5b33d
commit 15d5761ad3
24 changed files with 64 additions and 23 deletions
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14
Documentation/kbuild/makefiles.rst
View File

@@ -368,6 +368,14 @@ more details, with real examples.
subdir-ccflags-y := -Werror
ccflags-remove-y, asflags-remove-y
These flags are used to remove particular flags for the compiler,
assembler invocations.
Example::
ccflags-remove-$(CONFIG_MCOUNT) += -pg
CFLAGS_$@, AFLAGS_$@
CFLAGS_$@ and AFLAGS_$@ only apply to commands in current
kbuild makefile.
@@ -375,6 +383,9 @@ more details, with real examples.
$(CFLAGS_$@) specifies per-file options for $(CC). The $@
part has a literal value which specifies the file that it is for.
CFLAGS_$@ has the higher priority than ccflags-remove-y; CFLAGS_$@
can re-add compiler flags that were removed by ccflags-remove-y.
Example::
# drivers/scsi/Makefile
@@ -387,6 +398,9 @@ more details, with real examples.
$(AFLAGS_$@) is a similar feature for source files in assembly
languages.
AFLAGS_$@ has the higher priority than asflags-remove-y; AFLAGS_$@
can re-add assembler flags that were removed by asflags-remove-y.
Example::
# arch/arm/kernel/Makefile